This invention relates to an improved variable reactor for use in controlling the output voltage of a power supply such as described in U.S. patent application Ser. No. 070,479, filed on Aug. 28, 1979, in the name of Robert D. Peck and entitled "Power Supply". In such a supply, an unregulated DC voltage is produced by a rectifier coupled to the line and a chopper is coupled between the rectifier and a resonant circuit including the variable reactor. Regulation of the output voltage is achieved by varying the inductance of the reactor with power taken from the output. This is accomplished by passing current through a control winding that is mounted on the same core as the reactor winding. The power required is considerable in view of the fact that the core is gapped. Gapping is required for the following reason. At start-up, the inductance of the variable reactor has a maximum value because the core is unbiased and can restrict the power reaching the load to a point where it is insufficient to provide the current required in the control winding. This problem can be met even under the worst condition for start-up of minimum line voltage and maximum load by reducing the inductance with gaps in the core. Unfortunately, however, this may cause the maximum value of the inductance to be too low to produce the desired output voltage when the line voltage is a maximum and the load a minimum.